As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as a multi-gate field effect transistor (FET), including a fin FET (Fin FET) and a gate-all-around (GAA) FET. Field effect transistors (FETs) are widely used in integrated chips. FETs comprise a source, a drain, and a gate. A large subthreshold slope (i.e., a small subthreshold swing) is typically desired since it improves the ratio between on and off currents, and therefore reduces leakage currents.